Fixes two div by zero bugs in QUIC's BBR pacing code that caused server

net/quic/congestion_control/tcp_cubic_sender.cc

Index: net/quic/congestion_control/tcp_cubic_sender.cc
diff --git a/net/quic/congestion_control/tcp_cubic_sender.cc b/net/quic/congestion_control/tcp_cubic_sender.cc
index 0a0b6ae38bde0cd8344f5d0194f7639c893fd435..91b95cf29e85f2d6b692c81575bfc2dacc2f90f9 100644
--- a/net/quic/congestion_control/tcp_cubic_sender.cc
+++ b/net/quic/congestion_control/tcp_cubic_sender.cc
@@ -98,7 +98,7 @@ void TcpCubicSender::OnCongestionEvent(
     const CongestionVector& lost_packets) {
   if (rtt_updated && InSlowStart() &&
       hybrid_slow_start_.ShouldExitSlowStart(rtt_stats_->latest_rtt(),
-                                             rtt_stats_->min_rtt(),
+                                             rtt_stats_->MinRtt(),
                                              congestion_window_)) {
     slowstart_threshold_ = congestion_window_;
   }
@@ -212,6 +212,10 @@ QuicBandwidth TcpCubicSender::PacingRate() const {
 }
 
 QuicBandwidth TcpCubicSender::BandwidthEstimate() const {
+  if (rtt_stats_->SmoothedRtt().IsZero()) {
+    LOG(DFATAL) << "In BandwidthEstimate(), smoothed RTT is zero!";
+    return QuicBandwidth::Zero();
+  }
   return QuicBandwidth::FromBytesAndTimeDelta(GetCongestionWindow(),
                                               rtt_stats_->SmoothedRtt());
 }
@@ -300,7 +304,7 @@ void TcpCubicSender::MaybeIncreaseCwnd(
   } else {
     congestion_window_ = min(max_tcp_congestion_window_,
                              cubic_.CongestionWindowAfterAck(
-                                 congestion_window_, rtt_stats_->min_rtt()));
+                                 congestion_window_, rtt_stats_->MinRtt()));
     DVLOG(1) << "Cubic; congestion window: " << congestion_window_
              << " slowstart threshold: " << slowstart_threshold_;
   }